I accidentally deleted my .F3D design file. What to do now?
If you accidentally deleted any of your model (design) files in Fusion 360 (in the data panel), there is a way to recover and save them. This feature is only available to Administrators of the project.
Open your hub at myhub.autodesk360.com and verify that the file is indeed deleted and also not available in the archive.
In the given subfolder of the original file, click on the trash can icon to view the contents of the Trash folder (there is a distinct one for each project folder!):

In the trash can window, select the file you want to restore and choose "Restore" or "Delete forever" from the local menu (right mouse button) or from the trash can icons or back arrow displayed when you hover over the file:

The restored file will be returned to that project folder.
If the file is not in the trash folder, you can still try to save it from your local off-line cache (on the machine where it was used). You can find the cache folder on your computer (Windows) in C:\Users\USERNAME\AppData\Local\Autodesk\Autodesk Fusion 360\YOURFUSIONACCOUNTCODE\W.login\F.